1. Field of the Invention
This invention relates to the art of making electrical resistance spot welds in metal workpieces. More particularly, the invention pertains to the field of welding guns, capable of forcing the workpieces into mutual contact, holding them in position during welding, and forcing a welding electrode into contact with the workpieces.
2. Description of the Prior Art
The quality of an electrical resistance spot weld is adversely affected by poor fit of the workpieces to be welded if they are spaced from one another during welding rather than being in metal-to-metal contact. The spot welding gun used to make the weld must first pull the parts together into contact. But the force at the interface between the workpieces is less than the applied force because the workpieces themselves, due to their structural stiffness react the applied force and carry it from the welding zone. When the parts are particularly stiff and the space between them is sufficiently large, the clamping force applied by the welding gun can be so low at the welding zone that premature metal ejection and a weak weld will result. This condition is particularly serious where high strength steels are welded since these materials have a large stiffness modulus. Consequently, greater forces are required to produce the necessary defelection to bring the workpieces into contact at the welding gun. The loss of force at the welding zone is particularly deleterious in the case of coated or galvanized steels because ejection of metal between the sheets will occur.
When conventional force sensors are installed in standard welding guns, they expand and contract with thermal changes in the workpiece metal in the weld zone. Such sensors have been used to monitor the welding process and are described in U.S. Pat. No. 4,419,558 of Ariel Stiebel and in SAE Technical Paper 860579, February 1986. The device described in that patent does not use the workpiece surface itself as a reference surface. The device is therefore affected by changes in the thickness of the workpiece due to thermal effects and by the lack of metal-to-metal contact between the workpieces.